Pasteurization of foods is currently achieved by thermal treatment of the products to high temperatures. Such high temperatures are undesirable in many applications for they frequently lead to a degradation of the product in functionality and taste.
Pasteurization and sterilization of foods by thermal treatment have been in use for over one hundred years. Heating of a food kills the micro-organisms in the food which micro-organisms may degrade and eventually destroy the food within a short time. A reduction in the population of micro-organisms extends the shelf life of foods. Depending on the severity of thermal treatment given to the food, the shelf life may be extended many times. Other means of destroying micro-organisms or limiting their growth include irradiation, pickling, salting, sweetening, etc.
Pulsed high electric field treatment of liquid food has also been shown to kill bacteria by many researchers including the present inventor. The pulse duration of microseconds to milliseconds and field strengths of several kilovolts per centimeter have been found effective. However, the generation and application of such field have not yet been found effective and economical for industrial use.
For example, U.S. Pat. No. 5,514,391 May 7, 1996 (Bushnell) describes a process for reducing levels of microorganisms in pumpable food products using a high pulsed voltage system. The system includes a plurality of electric field treatment zones with cooling units between each pair of treatment zones. However, in this system, the electric field pulse is generated by electrical energy storage and fast discharge of this stored energy by complex switching circuits. The electric field pulse generators are expensive in capital and operating costs, and these are not competitive with thermal devices for pasteurization.
In high electric field pasteurization, bacteria are effectively electrocuted. If an electric field is used for ohmic or joule heating of the liquid, then the electric field is just a means of developing heat in the liquid. Bacterium killing takes place as a result of this heating. The present invention uses a pulsed high electric field to treat liquid food. A high enough voltage effectively electrocutes bacterial but short duration of pulses result in no appreciable heat generation in the food.